Conventionally, with an increase in communication data amount in wireless communication, an increase in transmission output of a base station has been demanded. As an amplifying apparatus for high frequency and high output of the base station, a GaN (gallium nitride) device may be used. The GaN device has wider bandgap and higher mobility than another semiconductor device (Si-LDMOS, GaAs-FET), thus having excellent high frequency and high output characteristics.
FIG. 10 is a diagram illustrating a configuration of a power amplifying apparatus using a conventional GaN device. As illustrated in FIG. 10, a power amplifying apparatus 200 has an input terminal 201, a DC (Direct Current) block 202, a GaN device 203, a variable resistance 204, and a gate resistance 205. The power amplifying apparatus 200 has a power supply terminal 206, a coil 207, a capacitor 208, a DC block 209 and an output terminal 210. An RF (Radio Frequency) signal input to the input terminal 201 is input to a gate of the GaN device 203 through the DC block 202. In the gate of the GaN device 203, a voltage VG is divided by the variable resistance 204 to supply the resultant as a gate voltage Vgs through the gate resistance 205.
On the other hand, power is supplied to the power supply terminal 206. The power is applied to a drain of the GaN device 203 through the coil 207 for high frequency interruption. The power supply terminal 206 is grounded through the capacitor 208 for a DC voltage element. This removes a high-frequency interference component when the component is applied to the power supply terminal 206. Moreover, the RF signal amplified by the GaN device 203 is output to the output terminal 210 via the DC block 209.
FIG. 11A is a diagram illustrating a relationship between an input power and a drain current in the power amplifying apparatus using the conventional GaN device. In FIG. 11A, the input power is defined on an x axis, and the drain current on a y axis. As illustrated in FIG. 11A, the relationship is shown, in which a drain current Ids of the GaN device increases with an increase in the input power, differing from an idling current. Here, the drain current is a current flowing between the drain and a source of the GaN device 203 in a state where a DC (Direct Current) voltage (e.g., 50 V) is applied to the power supply terminal 206, and is expressed by “Ids” in the following description. Moreover, the idling current is the drain current in an idling state (in a state where the RF signal is not input), and is particularly expressed by “Idq” in the following description in order to distinguish the idling current from the drain current Ids at the normal time.
FIG. 11B is a diagram illustrating a relationship between the gate voltage and the drain current when the input power is 0. In FIG. 11B, the gate voltage is defined on an x axis, and the drain current on a y axis. As illustrated in FIG. 11B, when the input power is 0, the drain current Ids of the GaN device varies in accordance with an increase in the gate voltage Vgs. In the power amplifying apparatus 200, the gate voltage Vgs is set to a prescribed value by adjustment of the variable resistance 204 so that properties such as linearity and efficiency of an output signal become optimal.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2001-320243
Patent Literature 2: Japanese Laid-open Patent Publication No. 2001-148615
Patent Literature 3: Japanese Laid-open Patent Publication No. 2010-74282
Patent Literature 4: Japanese Laid-open Patent Publication No. 2001-36351
In the power amplifying apparatus 200, the idling current Idg of the GaN device 203 is set to the prescribed value in advance by the adjustment of the variable resistance 204. However, since in the GaN device, there is a phenomenon of transient response of the drain current Ids, which is called Idq drift, the idling current Idg may largely fluctuate from the above-mentioned prescribed value, when the power amplifying apparatus 200 inputs the RF signal of the high power. The prescribed value is, for example, 400 mA to 800 mA.
FIG. 12A is a diagram illustrating chronological change in RF input signal intensity in the power amplifying apparatus using the conventional GaN device. In FIG. 12A, time t is defined on an x axis, and intensity of the RF input signal on a y axis. As illustrated in FIG. 12A, the RF signal of the high power is instantaneously input to the power amplifying apparatus 200 at time t4. FIG. 12B is a diagram illustrating chronological change of the idling current in the power amplifying apparatus using the conventional GaN device. In FIG. 12B, the time t is defined on an x axis, and the idling current on a y axis. As illustrated in FIG. 12B, while the idling current Idq is set to the prescribed value before the input of the RF signal, the idling current Idq instantaneously increases with the input of the RF signal of the high power. Thereafter, the idling current Idq enters an Idq drift state, exhibiting a behavior of largely decreasing, for example, to about 10% of the Idq prescribed value, and gradually returning to the above-mentioned prescribed value as time has further elapsed (e.g., about several seconds to one hour).
With the above-described fluctuation of the idling current, a gain of the GaN device also fluctuates. The above-described gain fluctuation causes problems in performance deterioration of distortion compensation or operation complication in operation of the apparatus configured by combining the power amplifying apparatus and a distortion compensating circuit.